On 2018-10-10 at 11:18:49 -0700, Dan Williams wrote: > On Wed, Oct 10, 2018 at 10:30 AM Michal Hocko <mhocko@xxxxxxxxxx> wrote: > > > > On Wed 10-10-18 09:39:08, Alexander Duyck wrote: > > > On 10/10/2018 2:58 AM, Michal Hocko wrote: > > > > On Tue 09-10-18 13:26:41, Alexander Duyck wrote: > > > > [...] > > > > > I would think with that being the case we still probably need the call to > > > > > __SetPageReserved to set the bit with the expectation that it will not be > > > > > cleared for device-pages since the pages are not onlined. Removing the call > > > > > to __SetPageReserved would probably introduce a number of regressions as > > > > > there are multiple spots that use the reserved bit to determine if a page > > > > > can be swapped out to disk, mapped as system memory, or migrated. > > > > > > > > PageReserved is meant to tell any potential pfn walkers that might get > > > > to this struct page to back off and not touch it. Even though > > > > ZONE_DEVICE doesn't online pages in traditional sense it makes those > > > > pages available for further use so the page reserved bit should be > > > > cleared. > > > > > > So from what I can tell that isn't necessarily the case. Specifically if the > > > pagemap type is MEMORY_DEVICE_PRIVATE or MEMORY_DEVICE_PUBLIC both are > > > special cases where the memory may not be accessible to the CPU or cannot be > > > pinned in order to allow for eviction. > > > > Could you give me an example please? > > > > > The specific case that Dan and Yi are referring to is for the type > > > MEMORY_DEVICE_FS_DAX. For that type I could probably look at not setting the > > > reserved bit. Part of me wants to say that we should wait and clear the bit > > > later, but that would end up just adding time back to initialization. At > > > this point I would consider the change more of a follow-up optimization > > > rather than a fix though since this is tailoring things specifically for DAX > > > versus the other ZONE_DEVICE types. > > > > I thought I have already made it clear that these zone device hacks are > > not acceptable to the generic hotplug code. If the current reserve bit > > handling is not correct then give us a specific reason for that and we > > can start thinking about the proper fix. > > > > Right, so we're in a situation where a hack is needed for KVM's > current interpretation of the Reserved flag relative to dax mapped > pages. I'm arguing to push that knowledge / handling as deep as > possible into the core rather than hack the leaf implementations like > KVM, i.e. disable the Reserved flag for all non-MEMORY_DEVICE_* > ZONE_DEVICE types. > > Here is the KVM thread about why they need a change: > > https://lkml.org/lkml/2018/9/7/552 > > ...and where I pushed back on a KVM-local hack: > > https://lkml.org/lkml/2018/9/19/154 Yeah, Thank Dan, I think I can going on with something like this: @@ -5589,6 +5589,7 @@ void __ref memmap_init_zone_device(struct zone *zone, struct page *page = pfn_to_page(pfn); __init_single_page(page, pfn, zone_idx, nid); + /* Could we move this a little bit earlier as I can + * direct use is_dax_page(page), or something else? + */ + page->pgmap = pgmap; /* * Mark page reserved as it will need to wait for onlining @@ -5597,14 +5598,14 @@ void __ref memmap_init_zone_device(struct zone *zone, * We can use the non-atomic __set_bit operation for setting * the flag as we are still initializing the pages. */ - __SetPageReserved(page); + if(!is_dax_page(page)) + __SetPageReserved(page); /* * ZONE_DEVICE pages union ->lru with a ->pgmap back * pointer and hmm_data. It is a bug if a ZONE_DEVICE * page is ever freed or placed on a driver-private list. */ - page->pgmap = pgmap; page->hmm_data = 0; After Alex's patch merged.
